At present, in the long-distance 100 G transmission in practical use, high-speed transmission of 128 G bps per channel is realized by setting the modulation symbol rate to approximately 32 G baud, by the use of a quadrature phase-shift keying (QPSK modulation) that is one type of multi-value modulation techniques and a polarization multiplexing that is one type of multiplexing techniques. (The transmission rate 28 G bps of the transmission rate 128 G bps is for the bits used for the information and error correction which are required for transmission control and thus the net data transmission rate becomes 100 G bps). In the further higher speed transmission, for example, the next-generation high-speed transmission such as 400 G or 1 T transmission, the introduction of a transmission technique for an optical multicarrier system using a plurality of optical transmission carriers is promising, in addition to the multivaluing of a modulation symbol and an increase in the modulation symbol rate. The optical multicarrier system is one type of wavelength division multiplexing technique or frequency division multiplexing technique, and achieves an increase in the channel capacity by handling each of a plurality of optical transmission carriers as a subchannel (subcarrier) and parallelizing the same. Therefore, a total channel transmission rate can be relatively easily enhanced from the viewpoint of easiness of realization, although there is a disadvantage that a plurality of devices is required in order to parallelize a plurality of optical transmission carriers. A large-capacity channel combining a plurality of such optical subcarriers is referred to also as a super channel.
In the multicarrier system, a multi-stage interferometer circuit serving as a multiplexer/demultiplexer for multiplexing/demultiplexing each subcarrier is required together with the parallelization of modulator/demodulator. This multi-stage interferometer circuit needs to satisfy the requirements such as low loss, a fully-circulating operation, a flat passband, multiport, and narrow channel spacing. The present examples of the multi-stage interferometer circuit that satisfies these requirements include a multi-stage interferometer circuit described in Patent Literature 1.
In recent years, there has attracted attention a Nyquist-shaping technique capable of performing the waveform shaping of a signal and narrowing a signal occupying bandwidth in order to enhance the spectrum usage efficiency of optical fiber transmission. In the ordinary modulation technique without performing waveform shaping, the width of signal light spectrum becomes twice the modulation symbol rate in terms of the width of a main lobe occupying the main power, and becomes twice or more the modulation symbol rate in terms of the width including the side lobe. In contrast, when the Nyquist-shaping technique is used, the width of a signal spectrum results in a highly rectangular spectrum that is narrow to the extent of the modulation symbol rate. Accordingly, as compared with the ordinary modulation technique, in a modulation signal using the Nyquist-shaping technique, carriers can be densely arranged and the spectrum usage efficiency of a transmission line can be enhanced. The same also applies to the arrangement of subcarriers in the above-described super channel. Namely, when the Nyquist-shaping technique is used in each subcarrier, the spectrum usage efficiency further increases since subcarriers can be more densely arranged and a total occupied bandwidth of a channel can be reduced.
Examples of the Nyquist-shaping techniques mainly include a method of using a digital signal processor (DSP) and a method of using an optical filter. The method using a DSP includes the steps of: calculating, in a digital domain, a signal waveform Nyquist-shaped by a transmitter; generating, with a digital-to-analog converter (DAC), the waveform thereof as an electric signal; and converting the electric signal to signal light with an optical modulator, thereby generating Nyquist-shaped signal light (for example, see Non-Patent Literature 1). The method of using an optical filter includes the step of band-limiting, in an analog manner, with an optical filter, signal light generated by an ordinary modulation technique, and correcting the spectrum shape thereof, thereby obtaining Nyquist-shaped signal light (for example, see Non-Patent Literatures 2 and 3).
The method of performing Nyquist-shaping with a DSP has, first of all, a problem that the power consumption of the DSP simply increases. Furthermore, a DSP in the transmitter is expected to have many functions such as multi-valuing of a signal and linear/non-linear prediction equalization of a transmission device and transmission line, other than the Nyquist-shaping function. On the other hand, in a high-speed DAC, the number of effective bits is approximately five, which cannot be said to be a sufficient number of bits for the purpose of waveform shaping. Accordingly, in particular, when all of the above-described functions are included, there is a problem that a sufficient feature of the waveform shaping function cannot be extracted. Moreover, in the method of using a DSP, the waveform shaping is to be performed for each signal light to be generated. Therefore, when there is a plurality of subcarrier signals as in the multicarrier system, there is a problem that, naturally, a DSP with the Nyquist-shaping function is required for each subcarrier and the power consumption increases.
In the method of using an optical filter, a steep and slightly complicated frequency characteristic is required for an optical filter to be used for Nyquist-shaping, and the characteristic adapted to the center wavelength of each channel is also required. Accordingly, a high-resolution variable filter using a spatial optical technique has been used for the optical filter, but such an optical filter has problems that it is expensive and the device size thereof is large, resulting in an increase in the scale of transmission equipment.
The present invention has been made in view of the above problems and the object thereof is to provide a multi-stage interferometer circuit, having a waveform-shaping optical filter function by the use of a waveguide-type technique, which is inexpensive and the size of which is small. Furthermore, the object thereof is also to provide a multi-stage interferometer circuit which has integrated therein multiplexing/demultiplexing functions required in the multicarrier system, and which is capable of multiplexing/demultiplexing and collectively waveform-shaping each subcarrier.